The present invention relates to a shadow mask type color picture tube and, more particularly, to a shadow mask-screen system thereof.
In a shadow mask-type color picture tube shown in FIG. 1, an envelope formed of glass substantially consists of a rectangular panel 1, a funnel 2 and a neck 3. On an inner surface of the panel 1, for example, a stripe phosphor screen 4 which emits red, green and blue light is provided. On the other hand, in-line electron guns 6, which are linearly arranged along a horizontal axis of the panel 1 and emit three electron beams 10 corresponding to red, green and blue, are provided in the neck 3. A shadow mask 5 having a main surface portion in which a plurality of apertures are formed is disposed adjacent and opposed to the screen 4. A peripheral portion of the shadow mask 5 has a skirt portion 8 which is bent in correspondence with an outer shape of the panel 1. The skirt portion 8 is supported and fixed by a mask frame 7 consisting of a frame having an L-shaped cross-section. Furthermore, the mask frame 7 is engaged through a spring 9 with a pin (not shown) which is buried in an inner wall of the panel 1. In such a color picture tube, the three electron beams 10 emit from the electron guns 6 are deflected by a deflection apparatus (not shown) provided near the funnel 2 of the outer portion of the envelope. The beams 10 are color-selected by the apertures of the shadow mask 5 while scanning a rectangular region substantially corresponding to the rectangular-shaped panel 1, and respectively and properly bombard on the corresponding color-emitting phosphor stripes, thereby forming a color image.
In this case, an effective amount of the electron beams 10 passing through the apertures of the shadow mask 5 is less than 1/3 of the total electron beams emitted from the electron guns 6. The remaining electron beams bombard the shadow mask 5 and are converted into heat energy. For this reason, the shadow mask 5 can be heated to about 80.degree. C. The shadow mask 5 comprises a thin plate having a thickness of 0.1 to 0.3 mm and is formed of cold-rolled steel mainly consisting of iron having a relatively large thermal expansion coefficient of 1.2.times.10.sup.-5 /.degree.C. The mask frame 7 which supports the skirt portion 8 of the shadow mask 5 is formed of the same cold-rolled steel as that of the shadow mask 5 and having a thickness of about 1 mm and an L-shaped cross-section. A surface of the mask frame 7 is oxidized, thereby forming a black oxide layer thereon. Thermal expansion of the shadow mask 5 which is heated by bombardment of the electron beams 10 can easily occur. However, since the peripheral portion of the shadow mask 5 is in contact with the mask frame 7 which has been subjected to darkening and has a large thermal capacity, heat is transferred to the mask frame 7 from the peripheral portion of the shadow mask 5 by radiation and conduction. Therefore, the temperature of the peripheral portion of the shadow mask 5 becomes lower than that of the central portion thereof. For this reason, a so-called doming occurs in which the central portion of the shadow mask 5 is thermally expanded by a greater extent than the peripheral portion thereof. By this doming, the relationship between the position of the apertures of the shadow mask 5 and that of the phosphor stripes formed corresponding to the apertures is changed. Therefore, a landing error occurs in which the electron beams 10 transmitting through the apertures do not bombard on the proper phosphor stripes, resulting in degradation of color purity. Particularly, this doming is considerable at the initial operating state of the color picture tube. When an image of partial high brightness is formed, the doming partially occurs at the shadow mask 5 in the same manner as described above.
With respect to such doming in the initial operating state of such a color picture tube, many suggestions have been made relating to the promotion of heat radiation from the central portion of a shadow mask or prevention of thermal conduction to the shadow mask. For example, in U.S. Pat. No. 2,826,538, it was proposed that a black layer formed of graphite be formed on a surface of a shadow mask so as to facilitate heat radiation of the shadow mask. In such a color picture tube, since this black layer serves as a good radiator, the temperature of the shadow mask is lowered. However, the black layer formed of graphite has the following drawbacks. Adhesion of the black layer is degraded due to a heat cycle in the heat treatment during the manufacturing process of the color picture tube. When vibration acts on the color picture tube, a part of the black layer is separated, thereby causing microparticles to drop off. If these particles of the black layer become attached to the shadow mask, the apertures formed thereon are closed, resulting in degradation of the image quality on the phosphor screen. On the other hand, if the particles become attached to the electron gun, a spark between electrodes is induced, thereby degrading the quality of the color picture tube, and, in particular, causing degradation of the break-down voltage.
In U.S. Pat. No. 3,887,828 as a second example, a color picture tube was proposed. In this color picture tube, a porous layer of manganese dioxide is deposited at a side of an electron gun of a shadow mask, and an aluminum layer and a nickel oxide or nickel-iron layer are sequentially formed thereon by vacuum evaporation. In this case, since the thermal conduction coefficient of the porous layer is extremely small, heat caused by bombardment of an electron beam is not transmitted to the shadow mask, but is radiated in a direction away from the shadow mask. For this reason, the temperature increase of the shadow mask can be effectively controlled. However, in order to provide three layers on a surface of the shadow mask, considerable equipment and operation time are needed, resulting in poor mass-producibility.